While valves of this kind to which the invention relates are commonly constructed with a body which is in a plurality of parts and which can be assembled about a corresponding valve member formed as a single integer, this construction presents various difficulties in manufacture and in operation, particularly with regard to obtaining proper alignment between and sealing between the parts of the body to secure satisfactory operation.
To avoid these difficulties it is known, for example from British patent specification No. 991,111, to construct the body in a single piece and to make the ball-shaped valve member in two discrete parts, which, in assembly of the valve, are passed separately through the inlet or outlet openings into the central chamber and are fitted together therein, after which the operating stem, formed separately from the valve member parts, is introduced, along said second axis, through a stem passage extending along said second axis and providing a bearing for the stem, sealing means being provided in said stem passage around the stem to prevent leakage from the valve along the stem passage.
With this arrangement, as the construction is such as to allow the stem to be introduced, during assembly, through the stem passage from the outside of the valve body, it is necessary to provide means for preventing ejection, or blow out, of the stem under pressure within the valve, and to design such means to be detachable for servicing of the valve for example for servicing the sealing means provided between the valve stem and the valve. Unfortunately, there is a danger, with such a design that if due care is not taken during servicing to ensure that the valve is turned off and isolated from fluid under pressure, removal, during servicing, of the means for preventing blow-out of the stem will result in the stem being ejected from the valve body by the fluid pressure within the valve, which is inconvenient and may even, depending on the conditions, be particularly dangerous.